EventSource

The EventSource interface is web content's interface to server-sent events. An EventSource instance opens a persistent connection to an HTTP server, which sends events in text/event-stream format. The connection remains open until closed by calling EventSource.close().

Once the connection is opened, incoming messages from the server are delivered to your code in the form of events. If there is an event field in the incoming message, the triggered event is the same as the event field value. If no event field is present, then a generic message event is fired.

Unlike WebSockets, server-sent events are unidirectional; that is, data messages are delivered in one direction, from the server to the client (such as a user's web browser). That makes them an excellent choice when there's no need to send data from the client to the server in message form. For example, EventSource is a useful approach for handling things like social media status updates, news feeds, or delivering data into a client-side storage mechanism like IndexedDB or web storage.

When not used over HTTP/2, SSE suffers from a limitation to the maximum number of open connections, which can be specially painful when opening various tabs as the limit is per browser and set to a very low number (6). The issue has been marked as "Won't fix" in Chrome and Firefox. This limit is per browser + domain, so that means that you can open 6 SSE connections across all of the tabs to www.example1.com and another 6 SSE connections to www.example2.com. (from Stackoverflow). When using HTTP/2, the maximum number of simultaneous HTTP streams is negotiated between the server and the client (defaults to 100).

Constructor

EventSource()
Creates a new EventSource to handle receiving server-sent events from a specified URL, optionally in credentials mode.

Properties

This interface also inherits properties from its parent, EventTarget.

EventSource.readyState Read only
A number representing the state of the connection. Possible values are CONNECTING (0), OPEN (1), or CLOSED (2).
EventSource.url Read only
A DOMString representing the URL of the source.
EventSource.withCredentials Read only
A Boolean indicating whether the EventSource object was instantiated with cross-origin (CORS) credentials set (true), or not (false, the default).

Event handlers

EventSource.onerror
Is an EventHandler called when an error occurs and the error event is dispatched on an EventSource object.
EventSource.onmessage
Is an EventHandler called when a message event is received, that is when a message is coming from the source.
EventSource.onopen
Is an EventHandler called when an open event is received, that is when the connection was just opened.

Methods

This interface also inherits methods from its parent, EventTarget.

EventSource.close()
Closes the connection, if any, and sets the readyState attribute to CLOSED. If the connection is already closed, the method does nothing.

Events

error
Fired when a connection to an event source failed to open.
message
Fired when data is received from an event source.
open
Fired when a connection to an event source has opened.

Additionally, the event source itself may send messages with an event field, which will create ad-hoc events keyed to that value.

Examples

In this basic example, an EventSource is created to receive unnamed events from the server; a page with the name sse.php is responsible for generating the events.

var evtSource = new EventSource('sse.php');
var eventList = document.querySelector('ul');

evtSource.onmessage = function(e) {
  var newElement = document.createElement("li");

  newElement.textContent = "message: " + e.data;
  eventList.appendChild(newElement);
}

Each received event causes our EventSource object's onmessage event handler to be run. It, in turn, creates a new <li> element and writes the message's data into it, then appends the new element to the list element already in the document.

Note: You can find a full example on GitHub — see Simple SSE demo using PHP.

To listen to named events, you'll require a listener for each type of event sent.

  const sse = new EventSource('/api/v1/sse');

  /* This will listen only for events 
   * similar to the following:
   * 
   * event: notice
   * data: useful data
   * id: someid
   *
   */
  sse.addEventListener("notice", function(e) { 
    console.log(e.data)
  })

  /* Similarly, this will listen for events 
   * with the field `event: update`
   */
  sse.addEventListener("update", function(e) {
    console.log(e.data)
  })

  /* The event "message" is a special case, as it
   * will capture events without an event field
   * as well as events that have the specific type
   * `event: message` It will not trigger on any
   * other event type.
   */
  sse.addEventListener("message", function(e) {
    console.log(e.data)
  })
  

Specifications

Specification Status
HTML Living Standard
The definition of 'EventSource' in that specification.
Living Standard

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
EventSourceChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support 11Safari Full support 5WebView Android Full support ≤37Chrome Android Full support 18Firefox Android Full support 45Opera Android Full support 11Safari iOS Full support 5Samsung Internet Android Full support 1.0
EventSource() constructorChrome Full support 9Edge Full support 79Firefox Full support 6IE No support NoOpera Full support 11Safari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
closeChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
error eventChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
message eventChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
onerrorChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
onmessageChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
onopenChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
open eventChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
readyStateChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
urlChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
withCredentialsChrome Full support 6Edge Full support 79Firefox Full support 6IE No support NoOpera Full support YesSafari Full support 5WebView Android Full support YesChrome Android Full support 18Firefox Android Full support 45Opera Android Full support 12Safari iOS Full support 5Samsung Internet Android Full support 1.0
Available in workersChrome Full support YesEdge Full support 79Firefox Full support 53IE No support NoOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesFirefox Android Full support 53Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yes

Legend

Full support  
Full support
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See also